1. Field of the Invention
The present invention relates to a galvannealed steel sheet having excellent formability and exfoliation resistance after adhesion and a production method thereof.
Priority is claimed based on Japanese Patent Application No. 2009-245872, filed on Oct. 26, 2009, the content of which is incorporated herein by reference.
2. Description of Related Art
A galvannealed steel sheet is excellent in characteristics such as coating adhesion, corrosion resistance after coating, weldability, and the like, and thus is widely used for automobiles, home electrical appliances, building materials, and the like. The galvannealed steel sheet is produced by performing hot dip galvanization of a steel sheet on the surface and immediately thereafter heating and maintaining the galvanized steel sheet at a temperature equal to or greater than the melting point of zinc and diffusing Fe from the steel sheet into zinc thereby forming a Zn—Fe alloy. Here, since the alloying rate varies significantly depending on the composition and structure of the steel sheet, in order to control the production process, highly advanced techniques are required. In addition, a steel sheet for an automobile which is pressed into a complex shape requires very high formability. In recent years, as the demand for corrosion resistance in automobiles has increased, cases where galvannealing are applied as steel sheets in automobiles has increased.
As the shapes of automobile bodies become complex, the demand on the formability of steel sheets has become stricter. Accordingly, better formability such as deep drawability than existing steel sheets is required of the galvannealed steel sheets.
For example, in Japanese Unexamined Patent Application, First Publication No. 59-74231 and Japanese Unexamined Patent Application, First Publication No. 59-190332, production methods are disclosed, which define compositions of a steel sheet, a hot rolling condition, and an annealing condition, producing a steel sheet having high ductility and a high r-value, and performing hot dipping on the surface of the steel sheet are disclosed. In addition, there may be a case where in order to enhance press formability and deep drawability of a galvannealed steel sheet, an oxide layer including phosphorus may be formed by treating a galvannealed surface of the steel sheet using a treatment liquid including phosphoric acid thereby providing the steel sheet with lubricity and an adhesion preventing property against a die.